ASP.NET logo

ASP.NET is a web application framework developed and marketed by Microsoft, that programmers can use to build dynamic web sites, web applications and web services. It was first released in January 2002 with version 1.0 of the .NET Framework, and is the successor to Microsoft's Active Server Pages (ASP) technology. ASP.NET is built on the Common Language Runtime (CLR), allowing programmers to write ASP.NET code using any supported .NET language.

History

After the release of Internet Information Services 4.0 in 1997, Microsoft began researching possibilities for a new web application model that would solve common complaints about Active Server Pages, especially with regard to separation of presentation and content and being able to write "clean" code. Mark Anders, a manager on the IIS team, and Scott Guthrie, who had joined Microsoft in 1997 after graduating from Duke University, were tasked with determining what that model would look like. The initial design was developed over the course of two months by Anders and Guthrie, and Guthrie coded the initial prototypes during the Christmas holidays in 1997.

Scott Guthrie in 2007.

The initial prototype was called "XSP"; Guthrie explained in a 2007 interview that, "People would always ask what the X stood for. At the time it really didn't stand for anything. XML started with that; XSLT started with that. Everything cool seemed to start with an X, so that's what we originally named it."^[1] The initial development of XSP was done using Java,^[3] but it was soon decided to build the new platform on top of the Common Language Runtime (CLR) instead. Guthrie described this decision as a "huge risk", as the success of their new web development platform would be tied to the success of the CLR, which, like XSP, was still in the early stages of development, so much so that the XSP team was the first team at Microsoft to target the CLR.

With the move to the Common Language Runtime, XSP was re-implemented in C# (known internally as "Project Cool" but kept secret from the public), and renamed to ASP+, as by this point the new platform was seen as being the successor to Active Server Pages, and the intention was to provide an easy migration path for ASP developers.

Mark Anders first demonstrated ASP+ at the ASP Connections conference in Phoenix, Arizona on May 2, 2000. Demonstrations to the wide public and initial beta release of ASP+ (and the rest of the .NET Framework) came at the 2000 Professional Developers Conference on July 11, 2000 in Orlando, Florida. During Bill Gates's keynote presentation, Fujitsu demonstrated ASP+ being used in conjunction with COBOL,^[5] and support for a variety of other languages was announced, including Microsoft's new Visual Basic .NET and C# languages, as well as Python and Perl support by way of interoperability tools created by ActiveState.

Once the ".NET" branding was decided on in the second half of 2000, it was decided to rename ASP+ to ASP.NET. Mark Anders explained on an appearance on The MSDN Show that year that, "The .NET initiative is really about a number of factors, it’s about delivering software as a service, it's about XML and web services and really enhancing the Internet in terms of what it can do .... we really wanted to bring its name more in line with the rest of the platform pieces that make up the .NET framework."

After four years of development, and a series of beta releases in 2000 and 2001, ASP.NET 1.0 was released on January 5, 2002 as part of version 1.0 of the .NET Framework. Even prior to the release, dozens of books had been written about ASP.NET, and Microsoft promoted it heavily as part of their platform for web services. Guthrie became the product unit manager for ASP.NET, and development continued apace, with version 1.1 being released on April 24, 2003 as a part of Windows Server 2003. This release focused on improving ASP.NET's support for mobile devices.

Characteristics

Pages

ASP.NET pages, known officially as "web forms", are the main building block for application development. Web forms are contained in files with an ASPX extension; in programming jargon, these files typically contain static (X)HTML markup, as well as markup defining server-side Web Controls and User Controls where the developers place all the required static and dynamic content for the web page. Additionally, dynamic code which runs on the server can be placed in a page within a block <% -- dynamic code -- %> which is similar to other web development technologies such as PHP, JSP, and ASP, but this practice is generally discouraged except for the purposes of data binding since it requires more calls when rendering the page.

Note that this sample uses code "inline", as opposed to code behind.

<%@ Page Language="C#" %>
 
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
 
<script runat="server">
 
    protected void Page_Load(object sender, EventArgs e)
    {
        Label1.Text = DateTime.Now.ToLongDateString();
    }
 
</script>
 
<html xmlns="http://www.w3.org/1999/xhtml" >
<head runat="server">
    <title>Sample page</title>
</head>
<body>
    <form id="form1" runat="server">
    <div>
         <form method="post" action="">
      <fieldset>
        <legend>Contact Form</legend>
        <label for="name">name: </label>
        <input type="text" id="nome" name="nome" /><br />
        <label for="surname">surname: </label>
        <input type="text" id="cognome" name="cognome" /><br />
        <label for="email">email: </label>
        <input type="text" id="email" name="email" /><br />
        <label for="password">password: </label>
        <input type="password" id="password" name="password" /><br />
        <label for="message">message: </label>
        <textarea id="message" name="message" rows="8" cols="20"></textarea><br />
        <input type="submit" id="go" value="send" />
     </fieldset>
    </form>
        <asp:Label runat="server" id="Label1" />
    </div>
    </form>
 
</body>
</html>

Code-behind model

It is recommended by Microsoft for dealing with dynamic program code to use the code-behind model, which places this code in a separate file or in a specially designated script tag. Code-behind files typically have names like MyPage.aspx.cs or MyPage.aspx.vb based on the ASPX file name (this practice is automatic in Microsoft Visual Studio and other IDEs). When using this style of programming, the developer writes code to respond to different events, like the page being loaded, or a control being clicked, rather than a procedural walk through the document.

ASP.NET's code-behind model marks a departure from Classic ASP in that it encourages developers to build applications with separation of presentation and content in mind. In theory, this would allow a web designer, for example, to focus on the design markup with less potential for disturbing the programming code that drives it. This is similar to the separation of the controller from the view in model-view-controller frameworks.

Example

<%@ Page Language="C#" CodeFile="SampleCodeBehind.aspx.cs" Inherits="Website.SampleCodeBehind" 
AutoEventWireup="true" %>

The above tag is placed at the beginning of the ASPX file. The CodeFile property of the @ Page directive specifies the file (.cs or .vb) that will act as the code-behind while the Inherits property specifies the Class the Page derives from. In this example, the @ Page directive is included in SamplePage.aspx, then SampleCodeBehind.aspx.cs will act as the code-behind for this page:

using System;
 
namespace Website
{
        public partial class SampleCodeBehind : System.Web.UI.Page
        {
                protected override void Page_Load(EventArgs e)
                {
                        base.OnLoad(e);
                }
        }
} 

In this case, the Page_Load() method will be called every time the ASPX page is requested. The programmer can implement event handlers at several stages of the page execution process to perform processing.

User controls

ASP.NET supports creating reusable components through the creation of User Controls. A User Control follows the same structure as a Web Form, except that such controls are derived from the System.Web.UI.UserControl class, and are stored in ASCX files. Like ASPX files, a ASCX contains static HTML or XHTML markup, as well as markup defining web control and other User Controls. The code-behind model can be used.

Programmers can add their own properties, methods, and event handlers. An event bubbling mechanism provides the ability to pass an event fired by a user control up to its containing page.

Rendering technique

ASP.NET uses a visited composites rendering technique. During compilation, the template (.aspx) file is compiled into initialization code which will build a control tree (the composite) representing the original template. Literal text goes into instances of the Literal control class, and server controls are represented by instances of a specific control class. The initialization code is combined with user-written code (usually by the assembly of multiple partial classes) and results in a class specific for the page. The page doubles as the root of the control tree.

Actual requests for the page are processed through a number of steps. First, during the initialization steps, an instance of the page class is created and the initialization code is executed. This produces the initial control tree which is now typically manipulated by the methods of the page in the following steps. As each node in the tree is a control represented as an instance of a class, the code may change the tree structure as well as manipulate the properties/methods of the individual nodes. Finally, during the rendering step a visitor is used to visit every node in the tree, asking each node to render itself using the methods of the visitor. The resulting HTML output is sent to the client.

After the request has been processed, the instance of the page class is discarded and with it the entire control tree.

State management

ASP.NET applications are hosted in a web server and are accessed over the stateless HTTP protocol. As such, if the application uses stateful interaction, it has to implement state management on its own. ASP.NET provides various functionality for state management in ASP.NET applications.

Application state

Application state is a collection of user-defined variables that are shared by all invocations of an ASP.NET application. These are set and initialized when the Application_OnStart event fires on the loading of the first instance of the applications and are available till the last instance exits. Application state variables are accessed using the Applications collection, which provides a wrapper for the application state variables. Application state variables are identified by names.

Session state

Session state is a collection of user-defined session variables, which are persisted during a user session. These variables are unique to different instances of a user session, and are accessed using the Session collection. Session variables can be set to be automatically destroyed after a defined time of inactivity, even if the session does not end. At the client end, a user session is identified either by a cookie or by encoding the session ID in the URL itself.

ASP.NET supports three modes of persistence for session variables:

In Process Mode

The session variables are maintained within the ASP.NET process. This is the fastest way, however, in this mode the variables are destroyed when the ASP.NET process is recycled or shut down. Since the application is recycled from time to time this mode is not recommended for critical applications.

ASPState Mode

In this mode, ASP.NET runs a separate Windows service that maintains the state variables. Because the state management happens outside the ASP.NET process, this has a negative impact on performance, but it allows multiple ASP.NET instances to share the same state server, thus allowing an ASP.NET application to be load-balanced and scaled out on multiple servers. Also, since the state management service runs independent of ASP.NET, variables can persist across ASP.NET process shutdowns.

SqlServer Mode

In this mode, the state variables are stored in a database server, accessible using SQL.Session variables can be persisted across ASP.NET process shutdowns in this mode as well. The main advantage of this mode is it would allow the application to balance load on a server cluster while sharing sessions between servers.

View state

View state refers to the page-level state management mechanism, which is utilized by the HTML pages emitted by ASP.NET applications to maintain the state of the web form controls and widgets.The state of the controls are encoded and sent to the server at every form submission in a hidden field known as __VIEWSTATE. The server sends back the variable so that when the page is re-rendered, the controls render at their last state. At the server side, the application might change the viewstate, if the processing results in updating the state of any control. The states of individual controls are decoded at the server, and are available for use in ASP.NET pages using the ViewState collection.

Template engine

When first released, ASP.NET lacked a template engine. Because the .NET framework is object-oriented and allows for inheritance, many developers would define a new base class that inherits from "System.Web.UI.Page", write methods here that render HTML, and then make the pages in their application inherit from this new class. While this allows for common elements to be reused across a site, it adds complexity and mixes source code with markup. Furthermore, this method can only be visually tested by running the application - not while designing it. Other developers have used include files and other tricks to avoid having to implement the same navigation and other elements in every page.

ASP.NET 2.0 introduced the concept of "master pages", which allow for template-based page development. A web application can have one or more master pages, which can be nested.^[14] Master templates have place-holder controls, called ContentPlaceHolders to denote where the dynamic content will go, as well as HTML and JavaScript that will be shared across child pages.

Child pages use those ContentPlaceHolder controls, which must be mapped to the place-holder of the master page that the content page is populating. The rest of the page is defined by the shared parts of the master page, much like a mail merge in a word processor. All markup and server controls in the content page must be placed within the ContentPlaceHolder control.

When a request is made for a content page, ASP.NET merges the output of the content page with the output of the master page, and sends the output to the user.

The master page remains fully accessible to the content page. This means that the content page may still manipulate headers, change title, configure caching etc. If the master page exposes public properties or methods (e.g. for setting copyright notices) the content page can use these as well.

Other files

Other file extensions associated with different versions of ASP.NET include:

Directory structure

In general, the ASP.NET directory structure can be determined by the developer's preferences. Apart from a few reserved directory names, the site can span any number of directories. The structure is typically reflected directly in the urls. Although ASP.NET provides means for intercepting the request at any point during processing, the developer is not forced to funnel requests through a central application or front controller.

The special directory names (from ASP.NET 2.0 on) are :

App_Browsers

holds site-specific browser definition files.

App_Code

This is the "raw code" directory. The ASP.NET server will automatically compile files (and subdirectories) in this folder into an assembly which is accessible in the code of every page of the site. App_Code will typically be used for data access abstraction code, model code and business code. Also any site-specific http handlers and modules and web service implementation go in this directory. As an alternative to using App_Code the developer may opt to provide a separate assembly with precompiled code.

App_Data

default directory for databases, such as Access mdb files and SQL Server mdf files. This directory is usually the only one with write access for the application.

App_LocalResources

Contains localized resource files for individual pages of the site. E.g. a file called CheckOut.aspx.fr-FR.resx holds localized resources for the french version of the CheckOut.aspx page. When the UI culture is set to french, ASP.NET will automatically find and use this file for localization.

App_GlobalResources

Holds resx files with localized resources available to every page of the site. This is where the ASP.NET developer will typically store localized messages etc. which are used on more than one page.

App_Themes

holds alternative themes of the site.

App_WebReferences

holds discovery files and WSDL files for references to web services to be consumed in the site.

Bin

Contains compiled code (.dll files) for controls, components, or other code that you want to reference in your application. Any classes represented by code in the Bin folder are automatically referenced in your application.

Performance

ASP.NET aims for performance benefits over other script-based technologies (including Classic ASP) by compiling the server-side code to one or more DLL files on the web server. This compilation happens automatically the first time a page is requested (which means the developer need not perform a separate compilation step for pages). This feature provides the ease of development offered by scripting languages with the performance benefits of a compiled binary. However, the compilation might cause a noticeable delay to the web user when the newly-edited page is first requested from the web server.

The ASPX and other resource files are placed in a virtual host on an Internet Information Services server (or other compatible ASP.NET servers; see Other Implementations, below). The first time a client requests a page, the .NET framework parses and compiles the file(s) into a .NET assembly and sends the response; subsequent requests are served from the DLL files. By default ASP.NET will compile the entire site in batches of 1000 files upon first request. If the compilation delay is causing problems, the batch size or the compilation strategy may be tweaked.

Developers can also choose to pre-compile their code before deployment, eliminating the need for just-in-time compilation in a production environment.

Extension

Microsoft has released some extension frameworks that plug into ASP.NET and extend its functionality. Some of them are:

ASP.NET AJAX

An extension with both client-side as well as server-side components for writing ASP.NET pages that incorporate AJAX functionality.

ASP.NET MVC Framework

An extension to author ASP.NET pages using the MVC architecture.

ASP.NET compared to ASP classic

ASP.NET attempts to simplify developers' transition from Windows application development to web development by offering the ability to build pages composed of controls similar to a Windows user interface. A web control, such as a button or label, functions in very much the same way as its Windows counterpart: code can assign its properties and respond to its events. Controls know how to render themselves: whereas Windows controls draw themselves to the screen, web controls produce segments of HTML and JavaScript which form part of the resulting page sent to the end-user's browser.

ASP.NET encourages the programmer to develop applications using an event-driven GUI paradigm (event-driven GUI model), rather than in conventional web-scripting environments like ASP and PHP. The framework attempts to combine existing technologies such as JavaScript with internal components like "ViewState" to bring persistent (inter-request) state to the inherently stateless web environment.

Other differences compared to ASP classic are:

• Compiled code means applications run faster with more design-time errors trapped at the development stage.
• Significantly improved run-time error handling, making use of exception handling using try-catch blocks.
• Similar metaphors to Windows applications such as controls and events.
• An extensive set of controls and class libraries allows the rapid building of applications, plus user-defined controls allow commonly used templates, such as menus. Layout of these controls on a page is easier because most of it can be done visually in most editors.
• ASP.NET leverages the multi-language capabilities of the .NET CLR, allowing web pages to be coded in VB.NET, C#, J#, Delphi.NET, Chrome etc.
• Ability to cache the whole page or just parts of it to improve performance.
• Ability to use the code-behind development model to separate business logic from presentation.
• If an ASP.NET application leaks memory, the ASP.NET runtime unloads the AppDomain hosting the erring application and reloads the application in a new AppDomain.
• Session state in ASP.NET can be saved in a SQL Server database or in a separate process running on the same machine as the web server or on a different machine. That way session values are not lost when the web server is reset or the ASP.NET worker process is recycled.
• Previous versions of ASP.NET (1.0 and 1.1) were criticized for their lack of standards compliance. The generated HTML and JavaScript sent to the client browser would not always validate against W3C/ECMA standards. In addition, the framework's browser detection feature sometimes incorrectly identified web browsers other than Microsoft's own Internet Explorer as "downlevel" and returned HTML/JavaScript to these clients with some of the features removed, or sometimes crippled or broken. However, in version 2.0, all controls generate valid HTML 4.0, XHTML 1.0 (the default) or XHTML 1.1 output, depending on the site configuration. Detection of standards-compliant web browsers is more robust and support for Cascading Style Sheets is more extensive.
• Web Server Controls: these are controls introduced by ASP.NET for providing the UI for the web form. These controls are state managed controls and are WYSIWYG controls.

Criticisms of ASP.NET

On IIS 6.0 and lower, pages written using different versions of the ASP framework can't share Session State without the use of third-party libraries. This criticism does not apply to ASP.NET and ASP applications running side by side on IIS 7. With IIS 7, modules may be run in an integrated pipeline that allows modules written in any language to be executed for any request.

ASP.NET 2.0 produces markup that passes W3C validation, but it is debatable as to whether this increases accessibility, one of the benefits of a semantic XHTML page + CSS representation. Several controls, such as the Login controls and the Wizard control, use HTML tables for layout by default. Microsoft has solved this problem by releasing the ASP.NET 2.0 CSS Control Adapters, a free add-on that produces compliant accessible XHTML+CSS markup.

Development tools

Several available software packages exist for developing ASP.NET applications:

• Delphi 2006
• Macromedia Dreamweaver MX, Macromedia Dreamweaver MX 2004, or Macromedia Dreamweaver 8 (doesn't support ASP.NET 2.0 features, and produces very inefficient code for ASP.NET 1.x: also, code generation and ASP.NET features support through version 8.0.1 was little if any changed from version MX: version 8.0.2 does add changes to improve security against SQL injection attacks)
• Macromedia HomeSite 5.5 (For ASP Tags)
• Microsoft Expression Web, part of the Microsoft Expression Studio application suite.
• Microsoft SharePoint Designer
• MonoDevelop (Free/Open Source)
• SharpDevelop (Free/Open Source)
• Visual Studio .NET (for ASP.NET 1.x)
• Visual Web Developer 2005 Express Edition (free) or Visual Studio 2005 (for ASP.NET 2.0)
• Visual Web Developer 2008 Express Edition (free) or Visual Studio 2008 (for ASP.NET 2.0/3.5)^[19]

Frameworks

It is not essential to use the standard webforms development model when developing with ASP.NET. Noteworthy frameworks designed for the platform include:

• Castle Monorail, an open-source MVC framework with an execution model similar to Ruby on Rails. The framework is commonly used with Castle ActiveRecord, an ORM layer built on NHibernate.
• Spring.NET, a port of the Spring framework for Java.
• Skaffold.NET, A simple framework for .NET applications, used in enterprise applications.